Study of the effect of hypothyroidism, induced by iodine deficiency or by propylthiouracil, on the initiation stage and on the growth stage of DMBA-induced mammary cancer in the rat. Investigation of the question of whether hypothyroidism augments DMBA-induced carcinogenesis through an effect on the absorption or degradation of the carcinogen, or whether it increases the intrinsic susceptibility of mammary tissue to the effect of the carcinogen. Study of the mechanism of the effect of hypothyroidism on the susceptibility of mammary tissue to DMBA-induced carcinogenesis. Investigation of the possible role of the marked increase in plasma TSH in hypothyroidism on the enhancement of mammary carcinogenesis. Study of the estrogen-dependence of DMBA-induced tumors developing in hypothyroid rats. Study of prolactin receptors in DMBA-induced tumors in hypothyroid rats.